pressure relief valve

I have a aosmith electic hot water heater and the pressure relief valve has a very slow leak. It take 3 days to get about a 1/4" of water in the pan. how hard is it to replace. Is it just a matter of screwing it out and putting a new one in.

Yes that's pretty much all there is to it. If it's an old tank it might be "frozen" in there pretty good. Be sure to kill the juice to the tank and drop the water level in the tank below the valve before you start.

It is not necessary to turn off the power to the tank, or drain it to replace the safety valve. Even if it is on the side of the tank, there will only be a small amount of water drain out if the replacement is done without dawdling around. Just turn off the water to the tank. Open a hot water faucet to relieve the pressure then close the faucet again before removing the leaking valve.

While the T&P valve could well be bad, it might just be doing what it is supposed to. Do you know what your water pressure is? Do you have a pressure reduction valve? Do you have an expansion tank? If so, is the tank shot? Does it have the proper pressure in it?

A leaking valve is a sign of over pressure, or overtemperature, or mineral deposits that prevent the thing from closing after testing. WHen the minerals build up, the spring is unable to reseat the shutoff.

If you have a pressure regulator valve, you have what is called a "closed system". What this means is that when water is heated, it expands. In an open system, this expanded water is absorbed by the water main. However, in a closed system, the pressure regulator valve prevents this expansion from taking place and the result is the pressure in the water heater rises sharply and trips the t/p valves to prevent the tank from blowing up. The cure for this problem is a relatively inexpensive expansion tank (about $50) that is placed in the supply line between the pressure regulator valve and the water heater. The expansion tank is air-charged to the same pressure as the regulator valve setting. This gives the expanding water somewhere to go.